In precision synchronization systems, a local time base reference of great accuracy and stability is required to generate and measure clock signals in a system within a certain accuracy. These time bases have normally crystal frequency sources; the characteristics of such devices are well known. In addition to variations in output frequency resulting from temperature and supply voltage changes, these devices have an ailing characteristic which implies that, over a long period of time, the output frequency will depart from the desired, nominal value. In order to maintain accuracies required in many telecommunication systems, either expensive oscillators must be used to overcome the aging effect or periodic calibration of the oscillators is required.
Calibration is often done manually; time base units are sent in to a calibration facility for extremely precise applications or can be field adjusted for less critical applications. In either case, the reference frequency source used for calibration must be more accurate than the time base unit to be calibrated. By measuring the adjustable time base with respect to the reference, the correction information can be obtained for adjusting the oscillator of the time base unit. The measurement equipment is extremely complicated and expensive, since it is designed to perform over a large range of frequencies. Additionally, the calibration procedure is expensive and usually requires the unit to be taken out of service.